Chemical mechanical polishing, also known as chemical mechanical planarization (referred to herein collectively as “CMP”), has been widely utilized for the planarization of semiconductor wafers. CMP produces a substantially smooth, planar face on one or more sides of a wafer. During CMP, an unprocessed wafer is typically first transferred to a work piece carrier head, which presses the wafer against a polish pad (or other polishing surface) supported by a platen assembly. Polishing slurry is introduced between the wafer's front surface and the polish pad, and relative motion (e.g., rotational, orbital, and/or linear) is initiated between the polish pad and the work piece carrier head. The mechanical abrasion of the polish pad and the chemical interaction of the slurry gradually remove topographical irregularities present on the wafer's front surface to produce a planar surface.
One known type of work piece carrier head comprises a housing having a flexible bladder coupled thereto, which contacts the back (i.e., the unpolished) surface of the wafer during polishing. The housing and the bladder cooperate to form a plurality of concentric pressure chambers or plenums behind the bladder. During CMP, the pressure within each of these plenums is independently adjusted to vary the force applied to the wafer's back surface by the bladder at different annular zones and consequently control the rate removal at different annular zones along the wafer's front surface. In this manner, the carrier head may compensate for topographical variations on wafer's front surface or other non-uniformities in the polishing process. For example, if a particular portion of wafer's front surface is determined to be relatively thick, the pressure within the corresponding plenum may be increased to intensify the rate of removal proximate the thicker area. Plenum pressure adjustments are typically performed by a closed-loop control (CLC) system, which may comprise a central CMP controller and a thickness measuring system.
To measure the thickness of the wafer during the polishing process, the CMP apparatus is typically equipped with a sensor system. One type of sensor system employs one or more eddy current probes, which induce and measure eddy currents in metal films indicative of the film thickness. Alternatively, the sensor system may employ optical probes that measure specific wavelengths of light in the visible spectrum, infrared, and/or ultraviolet spectrum to measure film thickness. The probes may be fixedly disposed within the platen assembly at different radial positions slightly below the polish pad. Each probe is electrically coupled to a sensor controller by way of a cable, which runs within a channel provided through the platen assembly. The sensor controller is operatively coupled to the central CMP controller and relays the film thickness readings thereto. The wafer readings are compiled to produce a topographical wafer map to which the central CMP controller may refer in determining appropriate plenum pressure adjustments.
Sensor systems of the type described above are limited in certain respects. For example, as each probe is fixedly disposed within the platen assembly, replacement or repair of a damaged probe may require disassembly of the entire platen assembly. Also, since each probe position is fixed, it can measure only a certain area of the wafer during processing. In addition, conventional sensor systems employ circuits supported by traditional printed circuit board (PCB) substrates, which may be damaged by vibrations produced during the CMP process. As another limitation, each probe is generally coupled to the sensor controller by way of a separate connector cable, which runs within a channel through the platen assembly. Each of these channels provides a potential leak path for polishing slurry and represents an unsupported region of the polish pad, which may dimple (i.e., become depressed) and lead to a non-uniform polishing. Finally, conventional sensor systems may be unable to measure other characteristics of the CMP polishing process (e.g., temperature, polish pressure, etc.) in addition to film thickness.
In view of the above, it should be appreciated that it would be desirable to provide a sensor system of the type employed in a platen assembly (or a work piece carried head) that overcomes the limitations associated with known sensor systems. Ideally, such a sensor system would employ at least one sensor capable of measuring the thickness of the film or wafer, which is vibration resistant and which could be replaced or repaired without disassembly of the platen assembly. If such sensor system employed multiple sensors, it would also be desirable if all of the sensors were coupled to the CMP controller via a single cable passing through a single channel through the platen assembly (thus minimizing the likelihood of slurry leakage and pad dimpling). Finally, it would be desirable if such a sensor system were capable of measuring characteristics of the CMP polishing process in addition to, or in lieu of, film thickness. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.